Pulmonary ventilation system and combination cardiac compressor and ventilation system



Aug. 19, 1969 c. E. BARKALOW 3 L ATION SYSTEM AND COMBINATION CARDIAC COMPRESSOR AND VENTILATION SYSTEM PULMONARY VENTIL Filed April 1'7, 3.96?v

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United States Patent 3,461,860 PULMONARY VENTILATION SYSTEM AND COMBINATION CARDIAC COMPRESSOR AND VENTILATION SYSTEM Clare E. Barkalow, Comstock Park, Mich., assignor to Michigan Instruments, Inc., Grand Rapids, Mich., a corporation of Michigan Filed Apr. 17, 1967, Ser. No. 631,317 Int. Cl. A6111 31/00, 31/02; A62]: 7/00; A61m 16/00 US. Cl. 128-53 14 Claims ABSTRACT OF THE DISCLOSURE A unique pulmonary ventilation system, preferably in combination with a cardiac compressor, having an automatically regulated, variable ventilation gas flow rate, regulated in response to feedback pressure from the patients pulmonary system. The oxygen enriched air output flow rate to the patient from a high volume, low pressure venturi pump is automatically regulated over a predetermined time period by a variable rate, flow control valve that regulates flow of oxygen to the venturi pump, in response to varying feedback pressure from the patients pulmonary system to the flow control valve.

Cross references to related applications This application is related to copending applications Ser. No. 409,634, filed Nov. 9, 1964 and now Patent No. 3,364,924, entitled Cardiac Compressor, and Ser. No. 584,402, filed Oct. 5, 1966, entitled Cardiac Compressor and Ventilation System, in each of which the inventor herein is a coinventor.

Background This invention relates to a pulmonary ventilation system, and preferably to a combination cardiac compressor and integrally associated ventilation system, and more particularly to such having improved variable ventilation flow rate control regulated by variable pressure feedback from the patient.

In this type of equipment, when in combination with the cardiac compressor, ventilation of the patient occurs by cycling the ventilator after a predetermined number of cycles of the cardiac compressor. During the ventilation cycle, the cardiac compressor is held inactive until completion of the ventilation cycle, to satisfy presently accepted medical techniques, at which time the cardiac compressor is again activated. Using the concepts set forth in the above identified applications, several advan tages are realized, including high ventilation flow rates from the venturi pump, beneficial use of exhaust oxygen from the cardiac compressor to operate the ventilator and to provide oxygen enriched air, and excellent synchronization and control of parameters between the cardiac compression and ventilation cycles.

In the previous apparatus, particularly as set forth in Ser. No. 584,402, the ventilating flow to the patient was cut off when a preset pressure was reached in the patients air system (or if this pressure was not reached, after a preset time interval had occurred). When the preset pressure or preset time occurred, the cardiac compressor was reactivated. This pressure cut-off could inadvertently prematurely occur by excessive initial flow to a patient having a pulmonary system with a low compliance or a significant airway resistance. In such a case, the back pressure cuts off further flow, although further flow is needed, and reactivates the cardiac compressor when the .lungs have only partially filled. While this prior apparatus had a manual flow rate adjust to enable the flow ice rate to be lowered to prevent premature cut-off and allows greater fill of the lungs before cut-off, it has been found that the natural reaction of the operator, even highly educated doctors and nurses, is to try to overcome the premature cut-off by actually increasing the flow rate rather than decreasing the flow rate as needed. This causes even more premature cut-01f. Further, it was found that such persons are extremely difficult to convince of the necessity of decreasing the flow rate for proper operation in such instances. Hence, for the good of the patients, the inventor herein determined that the apparatus should control the flow rate in a manner to obtain optimum results independent of the operator.

Summary of the invention It is therefore an object of this invention to provide a pulmonary ventilation system that automatically regulates the rate of air flow to the patient to obtain full, optimum ventilation, within controlled pressure limits, whether or not the patient has a restricted airway. The flow rate is automatically regulated over a wide flow rate range from maximum flow to no flow, in direct response to and in reverse proportion to back pressures in the patients airway. The ventilating pressure is maintained for a preset time interval, with accompanying, regulated, optimum flow rates from the maximum predetermined amount down to nil. The apparatus assures optimum lung filling at each ventilation cycle, and is believed to result in minimum development of atelectasis during the emergency treatment, reduction of previously existing atelectasis, reduction of arterial-venous shunting, and consequent reduction in arterial-venous admixture, and overall improvement in increased oxygen and reduced carbon dioxide levels in the blood.

Another object of this invention is to provide this novel pulmonary ventilation system in combination with a cardiac compressor.

The novel system automatically assures maximum permissible flow at a set pressure through any particular airway resistance, with maximum tidal volume during the ventilation period. The system is not volume limited as is true with all available equipment today. There is no need for manual control of flow rates, to cause confusion to the operator and potential detrimental results to the patient.

The system therefore has sustained ventilating pressure for a preset time interval, with flow rates being automatically regulated in response to back pressure from the patients airway, to accommodate the flow resistance of the particular patients airway for complete lung filling.

Brief description of the drawings FIG. 1 is a perspective view of the combination, without face mask and hose, showing the cardiac compressor portion with the ventilator control mechanism attached thereto;

FIG. 2 is a partially schematic, elevational view of the complete combination of this invention; and

FIG. 3 is a plan view of the ventilator control ap- 123131115 forming part of the mechanism illustrated in Description of the preferred embodiment The complete combination 10 of cardiac compressor subassembly 12 and ventilator subassembly 15 is preferably assembled in one integrally interconnected assembly for portability and easy manipulation as illustrated in FIG. 1. That is, preferably the cardiac compressor and the ventilator control is all interconnected except for an extended flexible hose 66 and face mask 58, or an endotracheal tube in place of the face mask. The flexible hose is connected to the air outlet 17 on the unit (FIG. 1).

The cardiac compressor subassembly 12 is basically like that shown in copending application Ser. No. 409,634, and like that shown in copending application Ser. No. 584,402. It includes a platform 16 for supporting the back of a patient, includes a removable upstanding columnar support 18, and overhanging beam or arm 20 mounted to support 1 8 with a releasable collar 20a, and having on the outer end thereof a cylinder 20' with a shiftable extendable plunger piston 21 and breastbone contacting pad 24. This plunger and pad are pneumatically operable to shift toward the platform 16 to compress the breastbone and thus the heart of the patient, and to return with expansion of his chest. Platform 16 has a thick hollow end 16' to which support 18 is removably mounted and which has an internal chamber 16a that encloses a control valve assembly 22 like that shown and explained in detail in the above identified copending patent applications, particularly with respect to FIG. 9 in Ser. No. 584,402, for controlling the repeat cycle of the cardiac compressor by causing repeat pulses of oxygen to be applied to the cardiac compressor and to be applied to the ventilator. Protruding from platform portion 16' is a pressure regulator knob 141 for the output from the control valve assembly to the compressor, and a corresponding pressure indicating gauge 41.

The cardiac compressor operates on a relatively high pulse rate. The ventilator operates on a much slower pulse rate, e.g. about to 1. When combining the two, means is provided to cause ventilation oxygen pulses only every multiple of compressor operating pulses. The compressor pulses are controlled by pulse controller 22. Only periodic output oxygen pulses of this controller are allowed to pass to the ventilator by programmer 52. The duration of each pulse to the ventilator is regulated by a timer control 516.

One of the advantages of this type of apparatus in combination is that both the cardiac compressor and the ventilator are pneumatically operated and also pneumatically controlled. The pressurized gas inlet hose 28 is connectable with an external source (not shown) of compressed gas, normally oxygen, and is connected to fixed connector 144 to supply the pressurized oxygen to unit 22 and for operating the entire system. This gas inlet hose to connector 44 therefore supplies the pressurized oxygen to the cycling compressor control valve assembly inside the cardiac compressor platform and thence to the air hose 40 that extends to the upper end of cylinder 30 to power the cardiac compressor. A manual shut off valve 40' may be provided on hose 40 to arrest the cardiac compressor manually while allowing the ventilator unit to still operate on its time cycled basis. Alternatively, the ventilator may be manually shut off using a valve 104a (FIG. 2) to arrest the ventilator while the cardiac compressor continues to operate, in a manner to be explained hereinafter.

The combination system also includes a programmer 52 of the type illustrated in detail in Ser. No. 584,402, and a diastolic delay control 516 of the type illustrated in detail in Ser. No. 584,402. These synchronize the compressor and the ventilator and regulate the interaction thereof. Programmer 52 is basically a pneumomechanical device which serves to periodically open a passageway for the flow of oxygen through it to the ventilator at regular intervals after a specific number of compression cycles of the cardiac compressor unit. Its control input through line 50 is a pulsing pressure bleed off from the conduit 40 to the cardiac compressor cylinder. The programmer can be preset to provide an open passageway at regular multiples of intervals, usually one out of five, of the cardiac compressor actuation, because the lungs should be ventilated only once every multiple of cardiac compressions. According to presently accepted medical teachings, it is normally set to open during cardiac diastole, i.e. compressor cylinder pressure of zero. When it opens the passageway, flow of oxygen occurs from the line 104 to lines 65 and 67. Line 104 is connected to primary oxygen supply means which normally also supplies the cardiac compressor, to enable the pressurized oxygen to be employed for the ventilation function.

As explained in detail in the above identified application Ser. No. 584,402, the function of diastolic delay control 516 is to temporarily restrain the cardiac compressor unit from operating while the ventilator is functioning on the patient. The basic cardiac compressor unit, the diastolic delay control unit 516, the programmer 52, and the cardiac compressor regulator valve 22 all operate basically in the same manner as the correspondingly enumerated items described in detail in copending application Ser. No. 584,402. However, whereas in the apparatus described in the above identified application, a pressure line extended from a cut-oil valve to the diastolic delay control for actuation and deactuation of the control, and thus control of the operation of the cardiac compressor, in this improved assembly, a cut-off valve is not used. Rather, a control pressure conduit 67 extends directly from programmer 52 to the diastolic delay control 516. The programmer 52, constitutes a valve that periodically opens after a predetermined number of pneumatic pulses are applied to it. After a predetermined number of input pulses are applied to it through line 61 corresponding to the pulsing of the cardiac compressor, the programmer allows primary oxygen to flow from line 104 through the programmer to line 65. At the same time, the programmer also causes a pressure pulse through line 67 to cause the diastolic delay control to close the passage between conduits 206' and 206 and prevent further actuation of the cardiac compressor until the preset delay period is completed, and the cardiac compressor is again acuated, thus cycling the programmer, which then cuts off oxygen flow to the ventilation.

This form of the apparatus, as before, also employs a venturi pump 56 having an oxygen inlet 460 and a surrounding air inlet space 163, for entrainment of the low pressure surrounding atmosphere by the high pressure, low volume oxygen, so that the outflow to the fiexible hose 66 and to the face mask 58 will be a high volume, low pressure, oxygen enriched ventilating gas. If desired, the pump can draw in exhaust oxygen from the compressor to enrich the gas out of the pump.

A unique feature of this system is the pressure responsive, flow rate control, valve unit 111. This unit includes a housing 113 for a spool valve 115, and another housing portion 117 for a shiftable diaphragm member 119. The spool valve has an inlet from line 67' and an outlet to line 123. The spool valve and diaphragm are positioned to abut each other on one end of the spool valve and one side of the diaphragm unit, so that, a force on the opposite end of the spool valve can shift the diaphragm actuator and the spool valve together, or alternatively, an overbalancing force on the other side of the diaphragm can shift the diaphragm and spool valve in the opposite direction. A pressure force on the left end of the spool valve, in the orientation illustrated, will cause the spool valve to move toward open position, while a pressure on the right side of the diaphragm, as the unit is illustrated, will cause the spool valve to move toward close position. The function of the unit is to control the rate of flow of primary oxygen which has passed through programmer 52 to conduit 65, through the control unit to the venturi pump. Conduit 6-7 is branched into the first branch which conducts the pressurized oxygen to unit 516, and to a second branch 67' which communicates with the end of the spool valve 115 opposite the diaphragm, to form a reactive pressure upon it. The force applied by the oxygen pressure on this end of the spool valve is equal to the pressure of the gas times the small area on the end of the spool. Such a force tends to shift the spool 115 toward full open position, allowing the pressurized oxygen to flow through the unit. Counteracting this force is a force created on diaphragm 119, equal to the substantially larger area of the diaphragm, times the low pressure differential between atmospheric pressure on the diaphragm back face and the pressure of the ventilating gas to the patient on the diaphragm front face. This latter pressure is actually the back pressure from the patients pulmonary system through face mask 58 and flexible tube 66 to the output of the venturi pump. This back pressure from the patient is sensed by a conduit 470 extending between the output of the venturi pump and the chamber adjacent the side of diaphragm 119 opposite the spool valve. The atmospheric pressure on the opposite diaphragm face is due to the vent 117' in housing 117.

The control unit 111 is capable of being preset to an initial zero position by a biasing compression spring 131 engaging diaphragm 119, and adjustable with a control knob 133 that is interconnected with the housing through a threaded stud, and with the spring by a plate 135. As will be realized, pressure in line 65, when it occurs, to the control unit, will tend to overcome the biasing force of spring 131 to open the spool valve and allow maximum flow. Reacting force on the diaphragm from the pressure feedback line 470 will tend to overcome the pressure on the end of the spool valve to throttle down the passage of compressed oxygen through the unit in relation to the increasing back pressure, so as to accurately regulate the flow of oxygen to the patient in inverse relation to the back pressure.

Since the operator of the equipment is interested in the pressure of the gas being applied to the patients pulmonary system, a pressure gauge may be mounted adjacent the flexible conduit 66. Alternatively, a pressure gauge 141 may be mounted on the high pressure side of unit 111, but calibrated in terms of the balanced pressure on the output from the venturi pump, since it is diflicult to obtain a pressure gauge capable of accurately indicating variations in the relatively low pressures which are existent at the face mask of the unit.

A pressure regulator 151 is provided in branch line 67', to enable regulation of the high pressure input to the end of the spool valve, thereby enabling adjustment of maximum pressure to be applied to the patient because of the regulation of the back pressure required to overcome the adjustable pressure on the spool valve. Also in this secondary line 67' is an adjustable orifice 153 which allows control of the build up of the valve biasing pressure and thus controlled rate of opening of the valve for regulating initial ventilation pressure at the start of each ventilating cycle. Further, the chamber adjacent the pressure end of the spool valve includes a small bleed orifice 155 to enable bleeding otf of the high pressure on the spool valve between the ventilator cycles. The purpose of these will be explained in more detail hereinafter.

In operation, after a patient has been positioned with his back on platform 16, and the compressor plunger positioned above his sternum and vertically and horizontally adjusted for optimum position, unit is activated as by throwing a toggle operated supply valve 161 (FIG. 1), making sure that the oxygen supply hose 28 is connected to connector 144. Knob 141 is adjusted slowly to controllably increase pressure to the cycling compressor plunger to apply the selected amount of force to the sternum for the size and anatomy of the patient, for compression of the heat on a timed cycle basis as controlled by the cardiac compressor regulator valve 22. Every time an input pulse of oxygen is sent by unit 22 to the cardiac compressor plunger, a like pulse is applied to line 50 to programmer 52. After a predetermined number of such pulses, usually five, the programmer allows flow of primary oxygen through line 104 and through programmer 52 to lines 65 and 67. The flow of oxygen through line 67 operates the diastolic delay control, to maintain the cardiac compressor inactive until the ventilation cycle is complete. Pressure in secondary conduit 67' biases the spool valve 115 to shift it toward open position against the bias of spring 131, allowing passage of oxygen from line 65 through control unit 111 to line 123 and thence to the venturi pump, where it causes air flow to also pass through the venturi, to provide to the patient a high volume, low pressure output from the high pressure, low volume oxygen input. With the mask 58 placed over the face of the patient, or with hose connected to an endotracheal tube inserted in the patients trachea, the ventilating, oxygen enriched air is forced into his pulmonary system.

The pressure of the ventilating gas constantly exerts a feedback pressure through line 470 to diaphragm 119. If the patient has unrestricted airways, this feedback pressure is relatively small until the lungs fill, thereby allowing valve to initially stay substantially open wide and allowing full flow of beneficial ventilating air to the patient.

If, on the other hand, the patient has a high resistance to flow in his airways, or has a low compliance pulmonary system, or both, the back pressure exerted into the breathing tube by the patients pulmonary system will be instantly transmitted back to the output zone of the venturi pump, and through line 470 to diaphragm actuator 119. This will shift the valve and automatically throttle the flow of gases through the flow rate regulator unit 111 in relation to the back pressure, so that the output of the unit will be inversely related to this back pressure. Yet, the flow of primary oxygen will continue to the venturi pump at the smaller rate for the entire predetermined period of ventilating time (which is controlled by diastolic delay control 516 as explained in the previous copending application identified above). The pump output will thus be automatically throttled, normally to a nil flow, ultimately. Yet, even if the lungs fill in a fraction of the time period allowed for ventilation, primary oxygen flow to the pump continues, to maintain the preset pressure on the patients pulmonary system for optimum ventilation even though the output from the pump may be nil. In other Words, the valve in unit 111 has a variable flow range from nil to maximum, and will automatically adjust during the entire preset time period of ventilation to maintain preset ventilation pressure and does not act to reactivate the cardiac compressor as occurred with the cut off valve of the system set forth in the earlier application. Further, there is no need for the operator to adjust the flow rate in order to obtain optimum ventilation since this is done automatically. Further, premature cut off of the ventilation cycle cannot occur.

Thus, for a fixed controlled pressure setting, a balance is achieved in primary oxygen flow, whereby secondary output flow is varied to maintain a constant limited airway pressure for a preset ventilation period. If for example, the input pressure is set to correspond to an output pressure of about 25 centimeters of water, then output flow will au tomatically adjust to provide that flow which will maintain a pressure of 25 centimeters of water into the particular airway situation that happens to exist. This pressure will be indicated on gauge 141. This operation will be done automatically, with airway pressures being controlled such that they cannot exceed the preset value. If airway resistance is low, initial flow will be relatively high. If resistance is high, initial flow will adjust to a corresponding lower value, to maintain a constant 25 centimeters of water pressure in the breathing hose as preset. Therefore, there is automatic assurance of maximum permissible flow at a set pressure through any particular airway resistance, and thus there is maximum tidal volume during the ventilation period. This feature, coupled with high potential flow capacity, provides in general for complete lung filling during ventilation. Thus, the system is not volume limited as is the case of all known equipment today. As stated previously, complete lung filling, physiologically, is believed to result in the reduction of development of atelectasis, the reduction of previously existing atelectasis, the reduction of arterial-venous shunting, the consequent reduction in arterial-venous admixture, and the overall improvement in increased oxygen and reduced carbon dioxide blood levels.

The preset pressure which is to be applied to the lungs is controlled by valve unit 151 on pressure line 67' to the spool valve. Obviously, if this pressure is adjusted to a smaller amount, the spool valve can be closed more readily by a smaller back pressure exerted on diaphragm 119.

The restricted orifice 153 in this pressure line 65" is to prevent an initial surge of high pressure oxygen from shifting spool valve 115 completely open during the initiation of the ventilation cycle. Such movement could cause an initial excessive pressure surge to be applied to the venturi pump and hence to the patients lungs in an undesirable manner. This control orifice causes the pressure to build up at a controlled rate, so that the pressure in the face mask is not applied too suddenly and does not surge above the ultimate desirable pressure and then return to the desired pressure, but rather gradually builds up to it. This creates a damping action for control of the system.

Also, bleed orifice 155 enables the spool actuating oxygen pressure to fall to ambient pressure during otf periods of the ventilator so that residual pressure will not remain on the high pressure side of the spool to cause an initial surge when ventilation is activated. The small leakage of primary can serve to further enrich the ventilatory air by allowing it to be drawn into the venturi pump.

Major usage of this ventilating system is intended to be beneficially employed in combination with the cardiac compressor. However, it is entirely possible that the ventilation could be employed apart from the compressor. It is only necessary that the regulator be supplied with cycling ventilator means of some type. Specifically, instead of the ventilation oxygen pulses being supplied by the combination (a) rapid rate compressor pulsor 22, (b) rate slowing programmer 52 which allows passage of only periodic oxygen pulses to the ventilator regulator, and (0) time period controller 51 6, only a simple timing pulse suppiler need be employed which would send relatively slow rate pressurized oxygen pulses to the regulator for predetermined time intervals.

It is also conceivable that certain variations in structure might be made without departing from the concept presented. Hence, it is intended that the invention is to be limited only by the scope of the appended claims, and the reasonably equivalent structures to those defined therein.

I claim:

1. In combination cardiac compressor and ventilator system having cycling cardiac compressor means, cycling ventilator means, programming means for activating said ventilator means for a cycle after every multiple of cardiac compressor means cycles, and said ventilator means having ventilating gas supply conduit means from said programmer to outlet means for a patient, the improvement comprising: a gaseous flow rate regulator in said conduit means having a varying flow rate capacity over a substantial flow rate range, means for applying a bias on said regulator towards maximum flow rate conditions, and pressure feedback means from said conduit means downstream of said regulator, back to said regulator, in opposition to said biasing means, to throttle the flow rate generally in relation to the increase in pressure in said conduit means downstream of said regulator, caused by patient airway resistance to flow and/ or low compliance of the patients pulmonary system.

2. In the combination in claim 1, said ventilator means also including a venturi pump in said conduit means downstream of said regulator, said conduit means upstream of said venturi pump including said regulator and being a relatively high pressure, low volume feed to said pump, and said conduit means downstream of said pump being a relatively low pressure, high volume discharge from said pump to said outlet means; and said pressure feedback means extending from downstream of said pump back to said regulator.

3. The combination in claim 2 wherein said cardiac compressor is pneumatically operated and has means to supply pneumatic pulses of compressed oxygen thereto, and wherein relatively high pressure low volume conduit means is also connected to said supply means, to cause oxygen flow from said supply means, through said programmer, through said regulator, and to said pump.

4. In a combination cardiac compressor and ventilator system having cycling, pneumatically operated, cardiac compressor means, cycling pneumatically supplied ventilator means, pneumatic-pulse-operated programming means operably associated with said cardiac compressor means to receive pneumatic pulses therefrom and operably associated with said ventilator means for activating the latter for a cycle after every multiple of cycles of said cardiac compressor means; pneumatic supply means from said programmer means with a gaseous flow rate regulator connected therewith to receive pressurized gas therefrom every ventilator cycle; said regulator having variable flow rate valving means in series with said pneumatic supply means and with an output from said regulator; secondary conduit means from said programmer means to said regulator valving means to apply a pneumatic biasing force on said valving means tending to shift it to open condition; said regulator output being connected to a venturi pump and breathing hose means and the latter having a pressure feedback to said regulator valving means in opposition to said biasing force to tend to shift said valving means to a closed condition, whereby said regulator controls flow rate therethrough in inverse function to said back pressure.

5. The combination in claim 4 including resilient biasing means biasing said regulator valving means to closed position between ventilation cycles, and restricted orifice means in said secondary conduit means from said programmer means to said regulator to prevent an initial pressure surge through said regulator with initiation of each ventilation cycle.

6. The combination in claim 5 including pressure control means in said secondary conduit means for varying said pneumatic biasing force and hence the back pressure necessary to overcome said pneumatic biasing force, and hence the maximum pressure to be applied to the patient.

7. The combination in claim 5 including restricted bleed orifice means to bleed said secondary conduit means and enable said resilient biasing means to close said valving means between ventilation cycles, prevent initial air flow pressure through said regulator from beginning at excessive valves with initiation of each ventilation cycle.

8. A pulmonary ventilation system having optimum ventilating gas flow at a regulated pressure, comprising: cycling ventilator means having ventilating-gas supply conduit means including outlet breathing hose means for a patient; a gaseous flow rate regulator in said conduit means having a varying flow rate capacity over a substantial flow rate range, means for applying a bias on said regulator towards maximum flow rate conditions, and pressure feedback means from said conduit means downstream of said regulator, back to said regulator, in opposition to said biasing means, to throttle the flow rate generally in relation to the increase in pressure in said conduit means downstream of said regulator, caused by patient airway resistance to flow and/or low compliance of the patients pulmonary system.

9. In the system in claim 8, said ventilator means also including a venturi pump in said conduit means downstream of said regulator, said conduit means upstream of said venturi pump including said regulator and being a relatively high pressure, low volume feed to said pump, and said conduit means downstream of said pump being a relatively low pressure, high volume discharge from said pump to said outlet means; and said pressure feedback means extending from downstream of said pump back to said regulator.

10. The system in claim 8 wherein: said regulator has variable rate valving means providing said varying fiow rate capacity, and has pressure responsive actuator means for said valving means; said pressure feedback means being operably associated with said actuator means.

11. The system in claim 10 wherein said conduit means includes secondary conduit means to said regulator for applying a pneumatic biasing force tending to shift said valving means to open condition during the ventilation cycles; resilient biasing means biasing said regulator valving means to closed position between ventilation cycles and of a strength to be overcome by said pneumatic biasing force during the ventilation cycles.

12. The system in claim 11 including restricted orifice means in said secondary conduit means to said regulator, to enable control of rate of valve opening during initiation of the ventilation cycle to prevent an initial pressure surge through said regulator with initiation of each ventilation cycle.

13. The system in claim 12 including pressure control means in said secondary conduit means for varying said pneumatic biasing force and hence the back pressure necessary to overcome said pneumatic biasing force, and hence the maximum pressure to be applied to the patient.

10 14. The system in claim 12 including restricted bleed orifice means to bleed said secondary conduit means and enable said resilient biasing means to close said valving means between ventilation cycles, and prevent initial air flow pressure through said regulator from beginning at excessive valves with initiation of each ventilation cycle.

References Cited UNITED STATES PATENTS 3,221,734 12/1965 Beasley 128145.8 3,254,645 6/1966 Rand et a1. 128-52 3,307,541 3/1967 Hewson 12853 L. W. TRAPP, Primary Examiner US. Cl. X.R. 128145.8 

